Basicity of water in dipolar aprotic solvents using the 1,4,8,11-tetramethyl-1,4,8,11-tetra-azacyclotetradecane nickel(II) cation as a probe of electron-pair acceptance

Abstract

The basicity of water in 1,2-dichloroethane (1,2-DCE), nitrobenzene (NB), nitromethane (NM), N,N-dimethylacetamide (DMA), and 1,2-DCE–DMA mixtures has been examined using spectral data of 1,4,8,11-tetramethyl-1,4,8,11-tetra-azacyclotetradecane nickel(II){[Ni(TMC)]2+}. The water molecules in 1,2-DCE, NB and NM, in which the monomeric water dominates, are barely coordinated to the cation, but in 1,2-DCE–DMA mixtures, in which the water molecules hydrogen bond with DMA molecules, they are easily coordinated. The equilibrium constant KNiw=[Ni(TMC)H2O2+]/[Ni(TMC)2+][H2O] was 1.9 in pure water, and 6.7 in a mixed solvent of 1.0 DMA fraction, 11.9 in 0.75, 23.2 in 0.50, 56.0 in 0.25 and 0 in 0. The results emphasize that hydrogen bonding of the water molecules with DMA increases the basicity of water molecules by polarization of O—H bonds and leads to water coordination. The monomeric water is not coordinated to the square-planar nickel(II) cation. I.r. spectra are presented to support these findings.